Use One2Many length to set sensitivity on next/previous buttons

Closes #12950

sao/src/view/form.js

@@ -3462,6 +3462,12 @@ function eval_pyson(value){
                 o2m_size = null;
                 size_limit = false;
             }
+            var first = false,
+                last = false;
+            if (typeof this._position == 'number') {
+                first = this._position <= 1;
+                last = this._position >= this._length;
+            }
             var deletable = this.screen.deletable;
             const view_type = this.screen.current_view.view_type;
             const has_views = this.screen.number_of_views > 1;
@@ -3497,12 +3503,13 @@ function eval_pyson(value){
             this.but_next.prop(
                 'disabled',
                 !record ||
-                (this.position > 0) &&
-                ( this._position >= this._length));
+                !this._length ||
+                last);
             this.but_previous.prop(
                 'disabled',
                 !record ||
-                this._position <= 1);
+                !this._length ||
+                first);
             if (this.attributes.add_remove) {
                 this.but_add.prop(
                     'disabled',

tryton/tryton/gui/window/view_form/view/form_gtk/one2many.py

@@ -318,10 +318,10 @@ class One2Many(Widget):
                 self._position
                 and self.read_access))
         self.but_next.set_sensitive(bool(
-                self._position
+                self._length
                 and not last))
         self.but_pre.set_sensitive(bool(
-                self._position
+                self._length
                 and not first))
         if self.attrs.get('add_remove'):
             self.but_add.set_sensitive(bool(